1. Field of the Invention
The present invention relates to a wireless communication apparatus and a wireless communication method for realizing broadband wireless transmission between a plurality of wireless stations, such as a wireless LAN (Local Area Network) or PAN (Personal Area Network). In particular, the invention relates to a wireless communication apparatus and a wireless communication method for expanding the transmission capacity by carrying out MIMO (Multi Input Multi Output) communication which uses spatial multiplexing and forms a plurality of logical channels, pairing a transmitter having a plurality of antennas with a receiver having a plurality of antennas.
More specifically, the invention relates to a wireless communication apparatus and a wireless communication method for solving the problem of a frequency error of a reception signal when a receiver performs MIMO synthesis on signals received from a plurality of antennas to spatially separate the signals into a plurality of orthogonal MIMO channels. In particular, the invention relates to a wireless communication apparatus and a wireless communication method for solving the problem of a frequency error and a timing drift when a receiver performs MIMO synthesis on signals received from a plurality of antennas to spatially separate the signals into a plurality of orthogonal MIMO channels in a MIMO communication system that employs multicarrier modulation.
2. Description of the Related Art
As a technology for realizing a higher speed of wireless communication, MIMO (Multi-Input Multi-Output) communication is coming to attention. This technology is for expanding transmission capacity and achieving improvement in communication speed by realizing spatially multiplexed transmission channels (hereinafter also referred to as “MIMO channels”) with a plurality of antenna elements at a transmitter and a receiver respectively.
In a MIMO communication system, a transmitter distributes transmission data to a plurality of antennas and transmits it through a plurality of virtual/logical MIMO channels, and a receiver obtains reception data by processing signals received by a plurality of antennas. In this manner, the MIMO communication system utilizes channel characteristics and differs from a simple transmission/reception adaptive array. The MIMO communication can increase the channel capacity according to the number of antennas without increase of the frequency band and accordingly has higher efficiency of frequency utilization.
FIG. 7 schematically shows the configuration of a MIMO communication system. As shown in FIG. 7, each of a transmitter and a receiver is equipped with a plurality of antennas. The transmitter space-time encodes a plurality of transmission data, multiplexes the encoded data, distributes the multiplexed signals to M antennas, and transmits them into a plurality of MIMO channels. The receiver receives the multiplexed transmission signals by N antennas through the MIMO channels and space-time decodes the received transmission signals to obtain reception data. In this case, the channel model is composed of a radio wave environment around the transmitter (transfer function), a structure of the channel space (transfer function), and a radio wave environment around the receiver (transfer function). Multiplexing the signals transmitted from the antennas involves crosstalk. However, it is possible to correctly extract the multiplexed signals without crosstalk through signal processing at the receiver.
Before transmitting the multiplexed signals, the MIMO transmitter transmits a training signal, e.g., for each antenna in a time-division manner, with which the receiver performs channel estimation. On the other hand, the MIMO receiver performs channel estimation at a channel estimation unit using training signals and calculates a channel information matrix H corresponding to the antenna pairs. Based on the channel information matrix H, the receiver improves the signal-to-noise ratio to enhance the degree of certainty of decoding.
Further, the MIMO transmitter space-time encodes a plurality of transmission data, multiplexes the encoded data, distributes the multiplexed signals to M antennas, and transmits them into a plurality of MIMO channels. The receiver receives the multiplexed transmission signals by N antennas through the MIMO channels and space-time decodes the received transmission signals to obtain reception data. Multiplexing the signals transmitted from the antennas involves crosstalk. However, it is possible to correctly extract the multiplexed signals without crosstalk through appropriate signal processing using the channel matrix at the receiver.
There are proposed a variety of methods for making up the MIMO transmission. However, it is an issue how channel information is exchanged between the transmitter and the receiver in accordance with an antenna configuration. MIMO transmission systems fall into two main types: an open-loop type of MIMO transmission system for performing spatial multiplexing transmission between the transmitter and the receiver being independent of each other, and as an extension of the open-loop type, a closed-loop type of MIMO transmission system for producing ideal spatial orthogonal channels between the transmitter and the receiver by feedback of channel information also from the receiver to the transmitter.
The open-loop type of MIMO transmission system can include V-BLAST (Vertical Bell Laboratories Layered Space Time) system (e.g., see patent document 1). The transmitter does not provide an antenna weighting coefficient matrix, but simply multiplexes a signal for each antenna and transmits. In this case, a feedback procedure for obtaining the antenna weighting coefficient matrix is all omitted.
Further, as an ideal form of the closed-loop type of MIMO transmission, there is known an SVD-MIMO system utilizing singular value decomposition (SVD) of a propagation function (e.g., see non-patent document 1). In the SVD-MIMO transmission, UDVH is obtained by performing the singular value decomposition of a numerical matrix whose elements denote channel information corresponding to respective antenna pairs, namely a channel information matrix H, and thus a transmission antenna weighting coefficient matrix V and a reception antenna weighting coefficient matrix UH are obtained. Thereby, each MIMO channel is expressed as a diagonal matrix D having the diagonal elements that are the square root of each eigenvalue λi, and signals can be multiplexed to be transmitted without any crosstalk. According to the SVD-MIMO transmission system, it is possible to realize a plurality of logically independent, spatially divided (i.e., spatially-multiplexed orthogonal) transmission channels at both the transmitter and the receiver. In theory, it is possible to achieve the maximum channel capacity. For example, if the transmitter and the receiver have two antennas each, it is possible to acquire double the transmission capacity at the maximum.
On the other hand, in the case of constructing a wireless network in a room, there is formed a multipath environment in which the receiver receives the superposition of direct waves and a plurality of reflected waves and delayed waves. A multipath produces delay distortion (or frequency selective fading), thereby causing an error in communication and interference between symbols.
Principal countermeasures against the delay distortion can include a multicarrier transmission system. In the multicarrier transmission system, transmission data is divided into multiple carriers having different frequencies for transmission. Accordingly, the bandwidth of each carrier becomes narrow, thereby being resistant to frequency selective fading.
For example, in an OFDM (Orthogonal Frequency Division Multiplexing) system which is one of the multicarrier transmission systems, the frequencies of carriers are set such that the carriers are orthogonal to each other in a symbol section. During information transmission, the transmitter converts information from serial to parallel form for each symbol period which is slower than an information transmission rate, assigns a plurality of converted data to each carrier, modulates the amplitude and phase of each carrier, transforms the modulated signals into time-domain signals while maintaining the orthogonality of each carrier in the frequency domain by performing an inverse FFT on the multiple carriers, and transmits the transformed signals. Further, during reception, as the inverse operations, the receiver transforms the time-domain signals to frequency-domain signals by performing an FFT, demodulates each carrier in accordance with each modulation scheme, and converts the demodulated signals from parallel to serial form to reproduce the information of an original serial signal.
IEEE802.11a/n which is a MIMO-transmission-applied LAN system adopts the OFDM modulation scheme.
In a general communication system, a preamble composed of known patterns is added to the head of a transmission frame (or packet) from the transmitter. Using the preamble, the receiver acquires synchronization and corrects a frequency offset to the transmitter. However, there is a problem of a residual frequency offset that an error remains in the case where an error arises in calculating a frequency offset due to noise etc.
In the case of a communication system to which the OFDM modulation scheme is applied, a frequency offset causes all subcarriers to rotate uniformly for each OFDM symbol. FIG. 8 shows in three dimensions the ratios between modulation points and subcarriers after channel correction on the phase space (constellation). The residual frequency offset is not so large that interference between subcarriers occurs. However, since the receiver merely corrects frequency offsets at the head (preamble) of a packet, as shown in FIG. 8 phase shifts are accumulated as OFDM symbols continue, thereby degrading the communication quality.
Further, in the multicarrier transmission system, there is a problem that a timing drift occurs during long continuous data symbols. A timing drift causes a twist of subcarrier phases. Since the receiver merely corrects frequency offsets at the head (preamble) of a packet, timing drifts are accumulated as OFDM symbols continue, and consequently the phase twist becomes larger as shown in FIG. 9, thereby degrading the communication quality. Further, both a frequency offset and a timing offset cause all subcarriers to rotate uniformly and twist, as shown in FIG. 10.
For example, in a SISO system in which data transmission is performed between a transmitter and a receiver each having a single antenna, it is possible to perform a phase track of a residual frequency estimation error of a reception signal using a pilot subcarrier (e.g., see non-patent document 2).
Further, in a multicarrier communication apparatus of the SISO type, by reproducing a reference phase and amplitude at the burst head, estimating a residual frequency offset from pilot information included in a detecting symbol and the preceding reference phase information, and generating reference phase information which is used at the time of detecting a symbol from the estimated residual frequency offset, it is possible to perform excellent demodulation (e.g., see patent document 4).
On the other hand, the MIMO receiver which synthesizes signals received from a plurality of antennas can perform synchronization acquisition and frequency correction using the preambles of the reception signals before MIMO synthesis; however, there is a problem that errors after frequency correction, i.e., residual frequency offsets are subjected to MIMO synthesis.
As described above, in the case of a long packet length, the MIMO-synthesized residual errors are accumulated as data symbols continue, thereby causing the phase rotation and phase twist which lead to errors. Especially in high modulation modes such as 64QAM and 256QAM, the communication is more susceptible to residual errors, thereby obstructing the achievement of high-throughput data transmission.
For example, there is proposed a wireless apparatus having a structure for compensating a frequency offset at the time of transmitting and receiving a signal with the MIMO system (e.g., see patent document 2). The wireless apparatus includes a plurality of antennas, a carrier oscillator which generates carriers for synchronization detection, multipliers which perform detection processing by multiplying a plurality of reception signals from a plurality of antennas by respective carriers, a frequency offset estimator which estimates one frequency offset based on the signals from the respective multipliers, and a frequency offset corrector which performs correction processing of the frequency offset on the signals from the respective multipliers based on a frequency offset estimate.
In the structure of the wireless apparatus, synchronization acquisition and frequency correction are performed before MIMO synthesis of the reception signals (i.e., spatial separation into each MIMO channel). The frequency offset estimator of the wireless apparatus is a common circuit for performing frequency offset estimation before MIMO synthesis, and no mention is made of how to process a residual estimation error after MIMO channel synthesis.
Further, there is proposed a wireless signal receiving apparatus which prevents the degradation of communication quality that occurs in the case of performing frequency correction by applying a carrier frequency error estimate obtained for each antenna route in the MIMO transmission (e.g., see patent document 3). In this case, the wireless signal receiving apparatus controls a carrier frequency error arising from multipath fading and thermal noise by obtaining a phase variation arising from the carrier frequency error after averaging autocorrelation values of pilot signals calculated for each antenna route, and provides commonality of the center frequencies of baseband signals among antenna routes by using the same carrier frequency correction value in all antenna routes, thereby improving the accuracy of the inverse function of a transfer function.
However, the wireless signal receiving apparatus extracts only pilot subcarriers having fixed phases before MIMO synthesis and estimates frequency errors by calculating the autocorrelation among OFDM symbols. In other words, residual components after the frequency correction are not treated; therefore, it may be impossible to eliminate the influence of phase rotations and phase twists by the MIMO-synthesized residual errors.
[Patent document 1] Japanese Patent Application Laid-Open No. 10-84324
[Patent document 2] Japanese Patent Application Laid-Open No. 2003-283359
[Patent document 3] Japanese Patent Application Laid-Open No. 2004-72458
[Patent document 4] Japanese Patent Application Laid-Open No. 13-69113
[Non-patent document 1] http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf (as of Oct. 24, 2003)
[Non-patent document 2] “802.11 high-speed wireless LAN textbook” by Matsue and Morikura (IDC Japan, IDC Information Series, 194 pages, First Edition: March 2003)